The present invention relates to a DC/DC converter for converting a DC input voltage of one magnitude into at least one DC output voltage of a different magnitude.
DC/DC converters are known which receive either a regulated or unregulated DC input (supply) voltage and produce one or more regulated DC output voltages of different magnitudes for powering electrical equipment. The purpose of the DC/DC converter is therefore (1) to produce DC voltages of the desired value and (2) to maintain these output voltages substantially constant under conditions of varying loads.
Countless circuits have been designed in the prior art to fulfill these functions. A chief objective in such design is to provide a circuit of sufficient capacity or rated power, at minimum cost.
Whereas, not so long ago, DC/DC converters were constructed of discrete components, integrated circuits are now available which carry out most of the incremental signal (control) functions required therein. Such integrated circuits offer substantial cost reductions in the design of the converter. Two such integrated circuits which operate as pulse generators for driving one or more power transistors are the SG3524 and the TL494.
Applications for the SG3524 are described in Elektronik Magazine, 1978, Issue 14; applications for the TL494 are described in a Texas Instruments application report entitled "Designing Switching Voltage Regulators with TL494", TI Bulletin CA-198.
The use of a standard, commercially available integrated circuit to reduce the cost of a DC/DC converter creates certain problems for the circuit designer, however. Since the standard IC is intended for wide commercial application, it does not contain the features necessary to fulfill the circuit requirements in every particular case.
For example, one of the problems encountered in designing a DC/DC converter with an integrated circuit is that the DC input supply voltage is either too high or too low to operate the integrated circuit, or the supply voltage is normally adequate to operate the chip but varies over a wide range which exceeds the limits on V.sub.cc prescribed by the integrated circuit manufacturer.
Another problem which arises in the use of an integrated circuit concerns the synchronization of this circuit to an external clock. The integrated circuit incorporates its own internal oscillator which may be timed by connecting an external resistor R.sub.T and an external capacitor C.sub.T to two respective pins on the IC package. There is, however, no convenient way to switch the operation of the integrated circuit from its internal oscillator (operating at the frequency prescribed by the time constant (R.sub.T C.sub.T) to an external oscillator.